Method and apparatus for detecting facet region, wafer producing method, and laser processing apparatus

ABSTRACT

A method of detecting a Facet region includes: a fluorescence luminance detecting step of detecting fluorescence luminance unique to SiC by irradiating a SiC ingot with exciting light having a predetermined wavelength from a top surface of the SiC ingot; and a coordinate setting step of setting a region in which the fluorescence luminance is equal to or higher than a predetermined value in the fluorescence luminance detecting step as a non-Facet region, setting a region in which the fluorescence luminance is lower than the predetermined value in the fluorescence luminance detecting step as a Facet region, and setting coordinates of a boundary between the Facet region and the non-Facet region.

This is a divisional application of application Ser. No. 16/675,798filed Nov. 6, 2019, which claims the benefit of Japanese PatentApplication No. 2018-210679, filed on Nov. 8, 2018.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method and an apparatus for detectingthe Facet region of a SiC ingot, a wafer producing method for producinga SiC wafer from the SiC ingot, and a laser processing apparatus forforming a peeling layer in the SiC ingot.

Description of the Related Art

Devices such as integrated circuits (ICs), large-scale integrations(LSIs), light emitting diodes (LEDs), or the like are formed bylaminating a functional layer to the top surface of a wafer whosematerial is Si (silicon), Al₂O₃ (sapphire), or the like, and demarcatingthe devices by a plurality of planned dividing lines intersecting thefunctional layer. In addition, power devices, LEDs, or the like areformed by laminating a functional layer to the top surface of a waferwhose material is hexagonal single crystal SiC (silicon carbide), anddemarcating the power devices, the LEDs, or the like by a plurality ofplanned dividing lines intersecting the functional layer. The wafer onwhich the devices are formed is divided into individual device chips byprocessing the planned dividing lines by a cutting apparatus or a laserprocessing apparatus. Each of the divided device chips is used in anelectric apparatus such as a mobile telephone, a personal computer, orthe like.

The wafer on which the devices are formed is generally produced bythinly cutting an ingot in a cylindrical shape with a wire saw. The topsurface and undersurface of the cut wafer are finished into a mirrorsurface by polishing (see Japanese Patent Laid-Open No. 2000-94221, forexample). However, when the ingot is cut by a wire saw, and the topsurface and undersurface of the cut wafer are polished, a large part(70% to 80%) of the ingot is discarded, which is uneconomical. The SiCingot, in particular, has a high hardness, and is difficult to cut witha wire saw. A considerable time is therefore taken to cut the SiC ingotwith a wire saw, thus resulting in poor productivity. In addition, theunit price of the ingot is high, and there is a problem in producing thewafer efficiently.

Accordingly, the present applicant has proposed a technology that formsa peeling layer in a planned cutting plane by irradiating a SiC ingotwith a laser beam having a wavelength transmissible through hexagonalsingle crystal SiC while positioning the focusing point of the laserbeam within the SiC ingot, and peeling off a SiC wafer from the SiCingot along the planned cutting plane in which the peeling layer isformed (see Japanese Patent Laid-Open No. 2016-111143, for example).

SUMMARY OF THE INVENTION

However, a region having a different crystal structure which region isreferred to as a Facet region may be present within the SiC ingot. TheFacet region has a high index of refraction and a high energy absorptionrate as compared with a non-Facet region. Thus, the position andfinished quality of the peeling layer formed within the SiC ingot by theapplication of the laser beam become nonuniform, and a level differenceoccurs between the Facet region and the non-Facet region in the wafer.

Accordingly, an object of the present invention is to provide a methodof detecting the Facet region of a SiC ingot which method can detect theFacet region and a non-Facet region.

Another object of the present invention is to provide an apparatus fordetecting the Facet region of a SiC ingot which apparatus can detect theFacet region and a non-Facet region.

Yet another object of the present invention is to provide a waferproducing method that can produce a wafer without a level differencebetween a Facet region and a non-Facet region.

Yet another object of the present invention is to provide a laserprocessing apparatus that can produce a wafer without a level differencebetween a Facet region and a non-Facet region.

In accordance with an aspect of the present invention, there is provideda method of detecting a Facet region of a SiC ingot, the methodincluding: a fluorescence luminance detecting step of detectingfluorescence luminance unique to SiC by irradiating the SiC ingot withexciting light having a predetermined wavelength from a top surface ofthe SiC ingot; and a coordinate setting step of setting a region inwhich the fluorescence luminance is equal to or higher than apredetermined value in the fluorescence luminance detecting step as anon-Facet region, setting a region in which the fluorescence luminanceis lower than the predetermined value in the fluorescence luminancedetecting step as a Facet region, and setting coordinates of a boundarybetween the Facet region and the non-Facet region.

In accordance with another aspect of the present invention, there isprovided a wafer producing method for producing a SiC wafer from a SiCingot, the wafer producing method including: a flat surface forming stepof forming a top surface of the SiC ingot into a flat surface bygrinding the top surface of the SiC ingot; a fluorescence luminancedetecting step of detecting fluorescence luminance unique to SiC byirradiating the SiC ingot with exciting light having a predeterminedwavelength from the top surface of the SiC ingot; a coordinate settingstep of setting, as an X-axis, a direction orthogonal to a direction inwhich a c-plane is inclined with respect to the top surface of the SiCingot and an off angle is formed, setting a direction orthogonal to theX-axis as a Y-axis, setting a region in which the fluorescence luminanceis equal to or higher than a predetermined value in the fluorescenceluminance detecting step as a non-Facet region, setting a region inwhich the fluorescence luminance is lower than the predetermined valuein the fluorescence luminance detecting step as a Facet region, andsetting X-coordinates and Y-coordinates of a boundary between the Facetregion and the non-Facet region; a processing feed step of forming aband-shaped peeling layer in which SiC is separated into Si and C and acrack extends along the c-plane, by positioning a focusing point formedby condensing a laser beam having a wavelength transmissible through SiCby a condenser at a depth corresponding to thickness of a wafer to beproduced from the top surface of the SiC ingot, and processing-feedingthe SiC ingot and the focusing point relative to each other in an X-axisdirection while irradiating the SiC ingot with the laser beam; anindexing feed step of arranging band-shaped peeling layers in a Y-axisdirection in parallel with each other by indexing-feeding the SiC ingotand the focusing point relative to each other in the Y-axis direction;and a peeling step of peeling off the wafer to be produced from thepeeling layers; the processing feed step increasing energy of the laserbeam and raising a position of the condenser at a time of irradiatingthe Facet region with the laser beam with respect to the energy of thelaser beam and the position of the condenser at a time of irradiatingthe non-Facet region with the laser beam on a basis of the X-coordinatesand the Y-coordinates of the boundary between the Facet region and thenon-Facet region, the X-coordinates and the Y-coordinates being set inthe coordinate setting step.

In accordance with a further aspect of the present invention, there isprovided an apparatus for detecting a Facet region of a SiC ingot, theapparatus including: fluorescence luminance detecting means detectingfluorescence luminance unique to SiC by irradiating the SiC ingot withexciting light having a predetermined wavelength from a top surface ofthe SiC ingot; and coordinate setting means setting a region in whichthe fluorescence luminance detected by the fluorescence luminancedetecting means is equal to or higher than a predetermined value as anon-Facet region, setting a region in which the fluorescence luminanceis lower than the predetermined value as a Facet region, and settingcoordinates of a boundary between the Facet region and the non-Facetregion.

In accordance with a still further aspect of the present invention,there is provided a laser processing apparatus for forming a peelinglayer in a SiC ingot, the laser processing apparatus including: aholding table configured to hold the SiC ingot; fluorescence luminancedetecting means detecting fluorescence luminance unique to SiC byirradiating the SiC ingot with exciting light having a predeterminedwavelength from a top surface of the SiC ingot; coordinate setting meanssetting, as an X-axis, a direction orthogonal to a direction in which ac-plane is inclined with respect to the top surface of the SiC ingot andan off angle is formed, setting a direction orthogonal to the X-axis asa Y-axis, setting a region in which the fluorescence luminance detectedby the fluorescence luminance detecting means is equal to or higher thana predetermined value as a non-Facet region, setting a region in whichthe fluorescence luminance is lower than the predetermined value as aFacet region, and setting X-coordinates and Y-coordinates of a boundarybetween the Facet region and the non-Facet region; a laser beamirradiating unit including a condenser that forms a peeling layer inwhich SiC is separated into Si and C and a crack extends along thec-plane, by positioning a focusing point of a laser beam having awavelength transmissible through SiC at a depth corresponding tothickness of a wafer to be produced from the top surface of the SiCingot, and irradiating the SiC ingot with the laser beam; an X-axisfeeding mechanism configured to processing-feed the holding table andthe condenser relative to each other in an X-axis direction; a Y-axisfeeding mechanism configured to indexing-feed the holding table and thecondenser relative to each other in a Y-axis direction; and a controlunit configured to increase energy of the laser beam and raise aposition of the condenser at a time of irradiating the Facet region withthe laser beam with respect to the energy of the laser beam and theposition of the condenser at a time of irradiating the non-Facet regionwith the laser beam on a basis of the X-coordinates and Y-coordinates ofthe boundary between the Facet region and the non-Facet region.

According to the Facet region detecting method in accordance with thepresent invention, the boundary between the Facet region and thenon-Facet region can be detected. Hence, on the basis of data on thedetected Facet region and the detected non-Facet region, processingconditions for irradiating the SiC ingot with the laser beam can becontrolled appropriately, so that the wafer without a level differencebetween the Facet region and the non-Facet region can be produced.

According to the wafer producing method in accordance with the presentinvention, the position and finished quality of the peeling layer formedin the Facet region and the non-Facet region become uniform, so that thewafer without a level difference between the Facet region and thenon-Facet region can be produced.

According to the Facet region detecting apparatus in accordance with thepresent invention, processing conditions for irradiating the SiC ingotwith the laser beam can be controlled appropriately on the basis of dataon the detected Facet region and the detected non-Facet region, so thatthe wafer without a level difference between the Facet region and thenon-Facet region can be produced.

According to the laser processing apparatus in accordance with thepresent invention, the position and finished quality of the peelinglayer formed in the Facet region and the non-Facet region becomeuniform, so that the wafer without a level difference between the Facetregion and the non-Facet region can be produced.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings depicting some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus accordingto an embodiment of the present invention;

FIG. 2 is a schematic diagram of fluorescence luminance detecting meansdepicted in FIG. 1;

FIG. 3 is a graph depicting relation between fluorescence wavelengths ofa Facet region and a non-Facet region and luminance in cases where thewavelength of exciting light is 370 nm and 273 nm;

FIG. 4 is a perspective view depicting a state in which a flat surfaceforming step is being performed;

FIG. 5A is a front view of a SiC ingot;

FIG. 5B is a plan view of the SiC ingot;

FIG. 6 is a perspective view depicting a state in which a fluorescenceluminance detecting step is being performed;

FIG. 7A is a schematic diagram of an image of the SiC ingot imaged inthe fluorescence luminance detecting step;

FIG. 7B is a table of X-coordinates and Y-coordinates of a boundarybetween the Facet region and the non-Facet region, the X-coordinates andY-coordinates being set in a coordinate setting step;

FIG. 8A is a perspective view depicting a state in which a processingfeed step is being performed;

FIG. 8B is a sectional view depicting the state in which the processingfeed step is being performed;

FIG. 9 is a sectional view depicting a peeling layer formed within theSiC ingot; and

FIG. 10 is a perspective view depicting a state in which a peeling stepis being performed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of a method and an apparatus for detecting a Facetregion, a wafer producing method, and a laser processing apparatusaccording to the present invention will hereinafter be described withreference to the drawings.

A laser processing apparatus according to an embodiment of the presentinvention will first be described with reference to FIG. 1. The laserprocessing apparatus indicated in entirety by reference numeral 2 isconstituted at least of: a holding unit 4 that holds a SiC ingot;fluorescence luminance detecting means 6 irradiating the SiC ingot withexciting light of a predetermined wavelength from the top surface of theSiC ingot and detecting fluorescence luminance unique to SiC; coordinatesetting means 8 setting a region in which the fluorescence luminancedetected by the fluorescence luminance detecting means 6 is equal to orhigher than a predetermined value as a non-Facet region, setting aregion in which the fluorescence luminance is lower than thepredetermined value as a Facet region, and setting the X-coordinates andY-coordinates of a boundary between the Facet region and the non-Facetregion; a laser beam irradiating unit 12 including a condenser 10 thatforms a peeling layer in which SiC is separated into Si and C and cracksextend along a c-plane, by positioning a focusing point of a laser beamhaving a wavelength transmissible through SiC at a depth correspondingto the thickness of a wafer to be produced from the top surface of theSiC ingot, and irradiating the SiC ingot with a laser beam; an X-axisfeeding mechanism 14 that processing-feeds the holding unit 4 and thecondenser 10 relative to each other in an X-axis direction; a Y-axisfeeding mechanism 16 that indexing-feeds the holding unit 4 and thecondenser 10 relative to each other in a Y-axis direction; and a controlunit 18 that controls the operation of the laser processing apparatus 2.Incidentally, the X-axis direction is a direction indicated by an arrowX in FIG. 1, and the Y-axis direction is a direction indicated by anarrow Y in FIG. 1 and is a direction orthogonal to the X-axis direction.In addition, a plane defined by the X-axis direction and the Y-axisdirection is substantially horizontal.

As depicted in FIG. 1, the holding unit 4 includes: an X-axis movableplate 22 mounted on a base 20 so as to be movable in the X-axisdirection; a Y-axis movable plate 24 mounted on the X-axis movable plate22 so as to be movable in the Y-axis direction; a holding table 26rotatably mounted on the top surface of the Y-axis movable plate 24; anda holding table motor (not depicted) that rotates the holding table 26.

The fluorescence luminance detecting means 6 will be described withreference to FIG. 1 and FIG. 2. The fluorescence luminance detectingmeans 6 according to the present embodiment is provided to a frame body28 that extends upward from the top surface of the base 20 and nextextends substantially horizontally. The fluorescence luminance detectingmeans 6 includes a case 30 fitted to an undersurface of an end of theframe body 28. In addition, as depicted in FIG. 2, the fluorescenceluminance detecting means 6 includes: a light source 32 that oscillatesexciting light EL having a low power (for example, 0.1 W) at such alevel that laser processing is not performed on the SiC ingot, andhaving a predetermined wavelength (for example, 370 nm); a dichroicmirror 34 that reflects the exciting light EL having the predeterminedwavelength which exciting light is oscillated from the light source 32and transmits light having a wavelength outside a first predeterminedwavelength range (for example, 365 to 375 nm) including theabove-described predetermined wavelength; a condensing lens 36 thatcondenses the exciting light EL reflected by the dichroic mirror 34 andirradiates the SiC ingot with the condensed exciting light EL; aband-pass filter 38 that transmits light in a second predeterminedwavelength range (for example, 395 to 430 nm); and a photodetector 40that detects the luminance of the light transmitted by the band-passfilter 38.

In the present embodiment, as depicted in FIG. 2, the light source 32,the dichroic mirror 34, the condensing lens 36, and the band-pass filter38 are arranged within the case 30. In addition, though not depicted,the fluorescence luminance detecting means 6 includes focusing pointposition adjusting means adjusting the vertical position of the focusingpoint of the exciting light EL by raising or lowering the case 30. Thefocusing point position adjusting means can be constituted of a ballscrew coupled to the case 30 and extending in a vertical direction and amotor for rotating the ball screw.

The exciting light EL emitted from the light source 32 is reflected bythe dichroic mirror 34, guided to the condensing lens 36, condensed inthe condensing lens 36, and applied to the SiC ingot. When the excitinglight EL is applied to the SiC ingot, fluorescence (radiated light) FLincluding a wavelength (for example, approximately 410 nm) differentfrom the wavelength of the exciting light EL is emitted from the SiCingot. The fluorescence FL passes through the condensing lens 36 and thedichroic mirror 34. Only the fluorescence FL in the second predeterminedwavelength range thereafter passes through the band-pass filter 38. Theluminance of the fluorescence FL passed through the band-pass filter 38is detected by the photodetector 40. The fluorescence luminancedetecting means 6 detects the luminance of the fluorescence FL unique toSiC on the entire top surface of the SiC ingot by irradiating the SiCingot with the exciting light EL having the predetermined wavelengthfrom the top surface of the SiC ingot while the SiC ingot and the case30 are moved relative to each other.

As depicted in FIG. 2, the coordinate setting means 8 is electricallyconnected to the photodetector 40 of the fluorescence luminancedetecting means 6. Data on the fluorescence luminance of each part ofthe SiC ingot which fluorescence luminance is detected by thephotodetector 40 is input to the coordinate setting means 8. Then, thecoordinate setting means 8 sets, as an X-axis, a direction orthogonal toa direction in which the c-plane is inclined with respect to the topsurface of the SiC ingot and an off angle is formed, sets a directionorthogonal to the X-axis as a Y-axis, sets, as a non-Facet region, aregion in which the fluorescence luminance detected by the fluorescenceluminance detecting means 6 is equal to or higher than the predeterminedvalue, sets a region in which the fluorescence luminance is lower thanthe predetermined value as a Facet region, and sets the X-coordinatesand Y-coordinates of a boundary between the Facet region and thenon-Facet region. Incidentally, the X-axis and the Y-axis used by thecoordinate setting means 8 are substantially identical to theabove-described X-axis direction and the above-described Y-axisdirection depicted in FIG. 1.

Here, description will be made of the predetermined value of theluminance as a determination criterion for the coordinate setting means8 to distinguish between the Facet region and the non-Facet region. Whenthe SiC ingot is irradiated with light having a wavelength of 370 nm ora wavelength of 273 nm as the exciting light EL, a luminance peak valueappears in the vicinity of 410 nm in the wavelengths of the fluorescenceFL emitted from the SiC ingot at either wavelength, as depicted in FIG.3. On the other hand, as is understood by reference to FIG. 3, luminancepeak values of the Facet region and the non-Facet region in the casewhere the wavelength of the exciting light EL is 370 nm are differentfrom luminance peak values of the Facet region and the non-Facet regionin the case where the wavelength of the exciting light EL is 273 nm.Hence, the predetermined value of the luminance as a determinationcriterion for distinguishing between the Facet region and the non-Facetregion is set so as to be between the luminance peak value of the Facetregion and the luminance peak value of the non-Facet region according tothe wavelength of the exciting light EL. It suffices for thepredetermined value to be about an intermediate value between theluminance peak value of the Facet region and the luminance peak value ofthe non-Facet region. In the case where the wavelength of the excitinglight EL is 370 nm, for example, the luminance peak value of the Facetregion is approximately 48 A. U. (see a thin solid line in FIG. 3), andthe luminance peak value of the non-Facet region is approximately 65 A.U. (see a thin dotted line in FIG. 3). Thus, the predetermined value canbe set to approximately 55 to 58 A. U. In addition, in the case wherethe wavelength of the exciting light EL is 273 nm, the luminance peakvalue of the Facet region is approximately 28 A. U. (see a thick solidline in FIG. 3), and the luminance peak value of the non-Facet region isapproximately 40 A. U. (see a thick dotted line in FIG. 3). Thus, thepredetermined value can be set to approximately 33 to 35 A. U.

As depicted in FIG. 1, the condenser 10 of the laser beam irradiatingunit 12 is fitted to the undersurface of the end of the frame body 28 atan interval in the X-axis direction from the case 30 of the fluorescenceluminance detecting means 6. In addition, though not depicted, the laserbeam irradiating unit 12 includes: a laser oscillator that oscillates apulsed laser having a wavelength transmissible through SiC; anattenuator that adjusts the power of the pulsed laser beam emitted fromthe laser oscillator; and focusing point position adjusting meansadjusting the vertical position of the focusing point of the pulsedlaser beam by raising or lowering the condenser 10. It suffices for thefocusing point position adjusting means to have a configurationincluding a ball screw coupled to the condenser 10 and extending in thevertical direction and a motor that rotates the ball screw.

In the laser beam irradiating unit 12, the condenser 10 is raised orlowered by the focusing point position adjusting means to position thefocusing point of the pulsed laser beam having the wavelengthtransmissible through SiC at a depth corresponding to the thickness of awafer to be produced from the top surface of the SiC ingot held by theholding unit 4, and then the pulsed laser beam emitted from the laseroscillator and adjusted in power by the attenuator is condensed by thecondenser 10 and applied to the SiC ingot. A peeling layer decreased instrength is thereby formed within the SiC ingot.

As depicted in FIG. 1, the X-axis feeding mechanism 14 includes a ballscrew 42 coupled to the X-axis movable plate 22 and extending in theX-axis direction and a motor 44 coupled to one end portion of the ballscrew 42. The X-axis feeding mechanism 14 converts a rotary motion ofthe motor 44 into a rectilinear motion by the ball screw 42 andtransmits the rectilinear motion to the X-axis movable plate 22, andthereby advances or retreats the X-axis movable plate 22 relative to thecondenser 10 in the X-axis direction along guide rails 20 a on the base20.

The Y-axis feeding mechanism 16 includes a ball screw 46 coupled to theY-axis movable plate 24 and extending in the Y-axis direction and amotor 48 coupled to one end portion of the ball screw 46. The Y-axisfeeding mechanism 16 converts a rotary motion of the motor 48 into arectilinear motion by the ball screw 46 and transmits the rectilinearmotion to the Y-axis movable plate 24, and thereby advances or retreatsthe Y-axis movable plate 24 relative to the condenser 10 in the Y-axisdirection along guide rails 22 a on the X-axis movable plate 22.

The control unit 18 is electrically connected to the coordinate settingmeans 8. The X-coordinates and Y-coordinates of the boundary between theFacet region and the non-Facet region which coordinates are set by thecoordinate setting means 8 are input to the control unit 18. The controlunit 18 increases the energy of the laser beam and raises the positionof the condenser 10 at a time of irradiating the Facet region with thelaser beam with respect to the energy of the laser beam and the positionof the condenser 10 at a time of irradiating the non-Facet region withthe laser beam on the basis of the X-coordinates and Y-coordinates ofthe boundary between the Facet region and the non-Facet region.Incidentally, while the control unit 18 and the coordinate setting means8 may be constituted by respective separate computers, the control unit18 and the coordinate setting means 8 may be constituted by a singlecomputer.

In the present embodiment, as depicted in FIG. 1, the laser processingapparatus 2 further includes: an imaging unit 50 that images the SiCingot held by the holding unit 4; a display unit 52 that displays animage imaged by the imaging unit 50; a grinding unit 54 that grinds thetop surface of the SiC ingot held by the holding unit 4; and a peelingmechanism 56 that peels off the wafer to be produced from the peelinglayer of the SiC ingot held by the holding unit 4.

The imaging unit 50 is fitted to the undersurface of the end of theframe body 28, and is disposed between the case 30 of the fluorescenceluminance detecting means 6 and the condenser 10 of the laser beamirradiating unit 12. In addition, the display unit 52 is disposed on thetop surface of the frame body 28.

The grinding unit 54 includes: a casing 58 fitted to a side surface ofthe frame body 28 so as to be movable in the Y-axis direction; a casingmoving mechanism 60 that moves the casing 58 in the Y-axis direction; anarm 62 extending in the Y-axis direction from a base end supported bythe casing 58 so as to be raisable and lowerable; arm raising andlowering means (not depicted) for raising and lowering the arm 62; and aspindle housing 64 fitted to an end of the arm 62.

The spindle housing 64 rotatably supports a spindle 66 extending in thevertical direction, and includes a spindle motor (not depicted) thatrotates the spindle 66. Making description with reference to FIG. 4, adisk-shaped wheel mount 68 is fixed to a lower end of the spindle 66,and an annular grinding wheel 72 is fixed by a bolt 70 to anundersurface of the wheel mount 68. A plurality of grinding stones 74annularly arranged at intervals in a circumferential direction are fixedto an outer circumferential edge portion of an undersurface of thegrinding wheel 72.

As depicted in FIG. 1, the peeling mechanism 56 includes: a casing 76disposed at terminal portions of the guide rails 20 a on the base 20; anarm 78 extending in the X-axis direction from a base end supported bythe casing 76 so as to be raisable and lowerable; and arm raising andlowering means (not depicted) for raising and lowering the arm 78. Amotor 80 is attached to an end of the arm 78. A suction piece 82 iscoupled to an undersurface of the motor 80 so as to be rotatable aboutan axis extending in the vertical direction. A plurality of suctionholes (not depicted) are formed in an undersurface of the suction piece82. The suction piece 82 is connected to suction means (not depicted).In addition, the suction piece 82 includes ultrasonic vibration applyingmeans (not depicted) for applying ultrasonic vibration to theundersurface of the suction piece 82.

FIG. 5A and FIG. 5B show a SiC ingot 84 formed of SiC. The SiC ingot 84as a whole is formed in a cylindrical shape. The SiC ingot 84 includes:a circular first end surface 86, a circular second end surface 88 on anopposite side from the first end surface 86; a peripheral surface 90located between the first end surface 86 and the second end surface 88;a c-axis (<0001> direction) extending from the first end surface 86 tothe second end surface 88; and a c-plane ({0001} plane) orthogonal tothe c-axis.

In the SiC ingot 84, the c-axis is inclined with respect to a normal 92to the first end surface 86, and an off angle α (for example, α=1, 3, 6degrees) is formed between the c-plane and the first end surface 86. Adirection in which the off angle α is formed is indicated by an arrow Ain FIG. 5. In addition, a rectangular first orientation flat 94 and arectangular second orientation flat 96 indicating a crystal orientationare formed on the peripheral surface 90 of the SiC ingot 84. The firstorientation flat 94 is parallel with the direction A in which the offangle α is formed. The second orientation flat 96 is orthogonal to thedirection A in which the off angle α is formed. As depicted in FIG. 5B,as viewed from above, a length L2 of the second orientation flat 96 isshorter than a length L1 of the first orientation flat 94 (L2<L1).

In addition, while the illustrated SiC ingot 84 is formed mainly of ahexagonal single crystal SiC, a Facet region 98 having a differentcrystal structure is locally present. The Facet region 98 is formed in acolumnar shape from the first end surface 86 to the second end surface88 of the SiC ingot 84, and is in a same shape in a thickness direction(vertical direction) of the SiC ingot 84 as in a Kintaro candy.Incidentally, a non-Facet region other than the Facet region 98 isindicated by reference numeral 100.

An embodiment of a wafer producing method according to the presentinvention will next be described. However, a wafer producing methodusing the above-described laser processing apparatus 2 will be describedin the following. In the present embodiment, first, the SiC ingot 84 isfixed on the top surface of the holding table 26 via an appropriateadhesive (for example, an epoxy resin-based adhesive). Incidentally, aplurality of suction holes may be formed in the top surface of theholding table 26, and the SiC ingot 84 may be sucked and held bygenerating a suction force in the top surface of the holding table 26.

After the SiC ingot 84 is fixed on the holding table 26, a flat surfaceforming step is performed in which the top surface of the SiC ingot 84is ground and formed into a flat surface, except for a case where a flattop surface of the SiC ingot 84 is already formed.

In the flat surface forming step, first, the holding table 26 ispositioned below the grinding wheel 72 of the grinding unit 54. Next, asdepicted in FIG. 4, the holding table 26 is rotated counterclockwise asviewed from above at a predetermined rotational speed (for example, 300rpm). In addition, the spindle 66 is rotated counterclockwise as viewedfrom above at a predetermined rotational speed (for example, 6000 rpm).Next, the grinding stones 74 are brought into contact with the topsurface of the SiC ingot 84 (first end surface 86 in the presentembodiment) by lowering the arm 62 by the arm raising and loweringmeans. The arm 62 is thereafter lowered at a predetermined grinding feedspeed (for example, 0.1 μm/s). Consequently, the top surface of the SiCingot 84 can be ground and formed into such a flat surface as not tohinder the incidence of the laser beam.

After the flat surface forming step is performed, a fluorescenceluminance detecting step is performed which detects fluorescenceluminance unique to SiC by irradiating the SiC ingot 84 with theexciting light EL having the predetermined wavelength from the topsurface of the SiC ingot 84.

In the fluorescence luminance detecting step, first, the holding table26 is positioned below the imaging unit 50, and the imaging unit 50images the SiC ingot 84 from the top surface thereof. Next, theorientation of the SiC ingot 84 is adjusted to a predeterminedorientation and the positions in the XY plane of the SiC ingot 84 andthe case 30 of the fluorescence luminance detecting means 6 are adjustedby moving and rotating the holding table 26 by the X-axis feedingmechanism 14, the Y-axis feeding mechanism 16, and the holding tablemotor on the basis of an image of the SiC ingot 84 imaged by the imagingunit 50. When the orientation of the SiC ingot 84 is adjusted to apredetermined orientation, a direction orthogonal to the direction A inwhich the off angle α is formed is aligned with the X-axis direction andthe direction A in which the off angle α is formed is aligned with theY-axis direction by aligning the second orientation flat 96 with theX-axis direction, as depicted in FIG. 6.

Next, the focusing point of the exciting light EL is positioned at anappropriate position (for example, the first end surface 86) of the SiCingot 84 by raising or lowering the case 30 by the focusing pointposition adjusting means. Next, the SiC ingot 84 is irradiated with theexciting light EL having a low power (for example, 0.1 W) at such alevel that laser processing is not performed on the SiC ingot 84, andhaving the predetermined wavelength (for example, 370 nm), while theX-axis feeding mechanism 14 moves the holding table 26 in the X-axisdirection aligned with the direction orthogonal to the direction A inwhich the off angle α is formed. Then, as depicted in FIG. 2,fluorescence (radiated light) FL including a wavelength (for example,approximately 410 nm) different from the wavelength of the excitinglight EL is emitted from the SiC ingot 84. The fluorescence FL passesthrough the condensing lens 36 and the dichroic mirror 34. Only thefluorescence FL in the second predetermined wavelength range (forexample, 395 to 430 nm) thereafter passes through the band-pass filter38. The luminance of the fluorescence FL passed through the band-passfilter 38 is detected by the photodetector 40.

Next, the SiC ingot 84 is indexing-fed relative to the focusing point ofthe exciting light EL in the Y-axis direction aligned with the directionA in which the off angle α is formed, by moving the holding table 26 bythe Y-axis feeding mechanism 16. Then, the irradiation with the excitinglight EL and the indexing feed are alternately repeated to detect, inassociation with an X-coordinate and a Y-coordinate, the luminance ofthe fluorescence FL in each of minute regions obtained by dividing thewhole of the first end surface 86 of the SiC ingot 84 into meshes of anappropriate size in the X-axis direction and the Y-axis direction. Dataon the luminance of the fluorescence FL detected by the photodetector 40is sent to the coordinate setting means 8 in association with theX-coordinates and the Y-coordinates.

In such a fluorescence luminance detecting step, the coordinate settingmeans 8 performs a coordinate setting step of setting, as the X-axis,the direction orthogonal to the direction A in which the c-plane isinclined with respect to the top surface of the SiC ingot 84 (first endsurface 86 in the present embodiment) and the off angle α is formed,setting the direction orthogonal to the X-axis as the Y-axis, setting,as the non-Facet region 100, a region in which the luminance of thefluorescence FL is equal to or higher than the predetermined value (forexample, approximately 55 to 58 A. U. in the case where the wavelengthof the exciting light EL is 370 nm), setting a region in which theluminance of the fluorescence FL is lower than the predetermined valueas the Facet region 98, and setting the X-coordinates and Y-coordinatesof the boundary between the Facet region 98 and the non-Facet region100. In the coordinate setting step in the present embodiment, thecoordinate setting means 8 sets a plurality of X-coordinates andY-coordinates (of 24 points from point a to point x, for example) of theboundary between the Facet region 98 and the non-Facet region 100, asdepicted in FIG. 7, on the basis of the data on the luminance of thefluorescence FL which data is sent from the photodetector 40 of thefluorescence luminance detecting means 6. Data on the plurality ofX-coordinates and Y-coordinates of the boundary between the Facet region98 and the non-Facet region 100 which coordinates are set by thecoordinate setting means 8 is sent to the control unit 18. Incidentally,the coordinate setting step may set the X-coordinates and Y-coordinatesof the entire Facet region 98 and set the X-coordinates andY-coordinates of the entire non-Facet region 100, and send theX-coordinates and Y-coordinates of the entire Facet region 98 and theX-coordinates and Y-coordinates of the entire non-Facet region 100 tothe control unit 18.

After the coordinate setting step is performed, a processing feed stepis performed which positions the focusing point formed by condensing thelaser beam having the wavelength transmissible through SiC by thecondenser 10 at a depth corresponding to the thickness of the wafer tobe produced from the top surface of the SiC ingot 84, processing-feedsthe SiC ingot 84 and the focusing point relative to each other in theX-axis direction while irradiating the SiC ingot 84 with the laser beam,and thereby forms a band-shaped peeling layer in which SiC is separatedinto Si and C and cracks extend along the c-plane.

In the processing feed step, first, the positions in the XY plane of theSiC ingot 84 and the condenser 10 are adjusted by moving the holdingtable 26 in the X-axis direction and the Y-axis direction on the basisof the image of the SiC ingot 84 imaged by the imaging unit 50 in thefluorescence luminance detecting step. Next, the condenser 10 is raisedor lowered by the focusing point position adjusting means to positionthe focusing point FP (see FIG. 8B) at the depth corresponding to thethickness of the wafer to be produced from the top surface of the SiCingot 84 in the non-Facet region 100. Next, as depicted in FIG. 8A, thepulsed laser beam LB having the wavelength transmissible through SiC isapplied from the condenser 10 to the SiC ingot 84 while the holdingtable 26 is moved at a predetermined feed speed in the X-axis directionaligned with the direction orthogonal to the direction A in which theoff angle α is formed. Then, as depicted in FIG. 9, a band-shapedpeeling layer 106 is formed along the X-axis direction in which peelinglayer SiC is separated into Si (silicon) and C (carbon) by theapplication of the pulsed laser beam LB, the pulsed laser beam LBapplied next is absorbed by C formed previously, and SiC is separatedinto Si and C in a chained manner, and also cracks 104 extendisotropically along the c-plane from a part 102 in which SiC isseparated into Si and C.

In such a processing feed step, the control unit 18 controls the laserbeam irradiating unit 12 so as to increase the energy of the pulsedlaser beam LB and raise the position of the condenser 10 at a time ofirradiating the Facet region 98 with the pulsed laser beam LB withrespect to the energy of the pulsed laser beam LB and the position ofthe condenser 10 at a time of irradiating the non-Facet region 100 withthe pulsed laser beam LB on the basis of the X-coordinates andY-coordinates of the Facet region 98 and the non-Facet region 100 whichcoordinates are set in the coordinate setting step. The index ofrefraction of the Facet region 98 is higher than the index of refractionof the non-Facet region 100. However, by performing control as describedabove, it is possible to make the depth of the focusing point FPsubstantially the same in the Facet region 98 and the non-Facet region100, and make the depth of the peeling layer 106 formed in the Facetregion 98 and the non-Facet region 100 substantially uniform, asdepicted in FIG. 8B. In addition, the Facet region 98 has a higherenergy absorption rate than the non-Facet region 100. However, finishedquality of the peeling layer 106 formed in the Facet region 98 and thenon-Facet region 100 can be made uniform by increasing the energy of thepulsed laser beam LB applied to the Facet region 98 as compared with theenergy of the pulsed laser beam LB applied to the non-Facet region 100.

Such a processing feed step can be performed under the followingprocessing conditions, for example. Incidentally, defocus in thefollowing is an amount of movement when the condenser 10 is moved towardthe top surface of the SiC ingot 84 from a state in which the focusingpoint FP of the pulsed laser beam LB is positioned at the top surface ofthe SiC ingot 84. (Non-Facet region: an index of refraction of 2.65)Wavelength of the pulsed laser beam: 1064 nm

Average power: 7 W

Repetition frequency: 30 kHz

Pulse width: 3 ns

Feed speed: 165 mm/s

Defocus: 188 μm

Position of the peeling layer from the top surface of the SiC ingot: 500μm (Facet region: an index of refraction of 2.79)

Wavelength of the pulsed laser beam: 1064 nm

Average power: 9.1 W

Repetition frequency: 30 kHz

Pulse width: 3 ns

Feed speed: 165 mm/s

Defocus: 179 μm

Position of the peeling layer from the top surface of the SiC ingot: 500μm

In addition, an indexing feed step is performed which arrangesband-shaped peeling layers 106 in the Y-axis direction in parallel witheach other by indexing-feeding the SiC ingot 84 and the focusing pointFP relative to each other in the Y-axis direction. In the presentembodiment, the above-described processing feed step is repeated whilethe SiC ingot 84 is indexing-fed relative to the focusing point FP inthe Y-axis direction by a predetermined indexing feed amount Li (seeFIG. 8A and FIG. 9). Consequently, band-shaped peeling layers 106extending in the X-axis direction can be arranged in the Y-axisdirection in parallel with each other within the SiC ingot 84. Inaddition, the peeling off of the wafer in the following peeling step isfacilitated by setting the indexing feed amount Li in a range notexceeding the width of the cracks 104, and making the cracks 104 ofpeeling layers 106 adjacent to each other in the Y-axis directionoverlap each other as viewed in the vertical direction.

After a plurality of band-shaped peeling layers 106 are formed in theSiC ingot 84 by performing the processing feed step and the indexingfeed step, a peeling step is performed which peels off the wafer to beproduced from the peeling layer 106. In the peeling step, first, theholding table 26 is moved to a position below the suction piece 82 ofthe peeling mechanism 56. Next, the arm 78 is lowered by the arm raisingand lowering means to bring the undersurface of the suction piece 82into close contact with the first end surface 86 of the SiC ingot 84, asdepicted in FIG. 10. Next, the undersurface of the suction piece 82 isstuck to the first end surface 86 of the SiC ingot 84 by actuating thesuction means. Next, ultrasonic vibration is applied to the undersurfaceof the suction piece 82 by actuating the ultrasonic vibration applyingmeans, and the suction piece 82 is rotated by the motor 80. A SiC wafer108 to be produced can be thereby peeled off from the peeling layer 106.

In addition, by flattening a peeling surface by subjecting the SiC ingot84 from which the SiC wafer 108 is peeled off to the above-describedflat surface forming step, and thereafter repeating the processing feedstep, the indexing feed step, and the peeling step, it is possible toproduce a plurality of SiC wafers 108 from the SiC ingot 84. As for thefluorescence luminance detecting step and the coordinate setting step,because the Facet region 98 is formed in a columnar shape from the topsurface to the undersurface of the SiC ingot 84 and has a same shape asin a Kintaro candy in the thickness direction, it suffices to performthe fluorescence luminance detecting step and the coordinate settingstep when the first SiC wafer 108 is produced from the SiC ingot 84, andthe fluorescence luminance detecting step and the coordinate settingstep do not have to be performed when a second and subsequent SiC wafers108 are produced.

As described above, in the present embodiment, the position and finishedquality of the peeling layers 106 formed in the Facet region 98 and thenon-Facet region 100 can be made uniform. It is therefore possible toproduce the SiC wafer 108 without a level difference between the Facetregion 98 and the non-Facet region 100. Hence, it is not necessary toallow for the level difference between the Facet region 98 and thenon-Facet region 100 and peel off a thick SiC wafer 108. An improvementin efficiency can therefore be achieved.

Incidentally, while description has been made of an example in which thefluorescence luminance detecting means 6 and the coordinate settingmeans 8 are incorporated in the laser processing apparatus 2 in theforegoing present embodiment, the fluorescence luminance detecting means6 and the coordinate setting means 8 may not be incorporated in thelaser processing apparatus 2. That is, the fluorescence luminancedetecting means 6 and the coordinate setting means 8 may be constituentelements of a Facet region detecting apparatus including at least thefluorescence luminance detecting means 6 and the coordinate settingmeans 8. Then, using the Facet region detecting apparatus including atleast the fluorescence luminance detecting means 6 and the coordinatesetting means 8, a Facet region detecting method including at least thefluorescence luminance detecting step and the coordinate setting stepdescribed above may be performed. It is thereby possible to detect theFacet region 98 and the non-Facet region 100 of the SiC ingot 84. Thus,on the basis of data on the detected Facet region 98 and the detectednon-Facet region 100, processing conditions for irradiating the SiCingot 84 with the pulsed laser beam LB can be controlled appropriately,so that the SiC wafer 108 without a level difference between the Facetregion 98 and the non-Facet region 100 can be produced.

The present invention is not limited to the details of the abovedescribed preferred embodiments. The scope of the invention is definedby the appended claims and all changes and modifications as fall withinthe equivalence of the scope of the claims are therefore to be embracedby the invention.

What is claimed is:
 1. An apparatus for detecting a Facet region of aSiC ingot, the apparatus comprising: fluorescence luminance detectingmeans detecting fluorescence luminance unique to SiC by irradiating theSiC ingot with exciting light having a predetermined wavelength from atop surface of the SiC ingot; and coordinate setting means setting aregion in which the fluorescence luminance detected by the fluorescenceluminance detecting means is equal to or higher than a predeterminedvalue as a non-Facet region, setting a region in which the fluorescenceluminance is lower than the predetermined value as a Facet region, andsetting coordinates of a boundary between the Facet region and thenon-Facet region.
 2. A laser processing apparatus for forming a peelinglayer in a SiC ingot, the laser processing apparatus comprising: aholding table configured to hold the SiC ingot; fluorescence luminancedetecting means detecting fluorescence luminance unique to SiC byirradiating the SiC ingot with exciting light having a predeterminedwavelength from a top surface of the SiC ingot; coordinate setting meanssetting, as an X-axis, a direction orthogonal to a direction in which ac-plane is inclined with respect to the top surface of the SiC ingot andan off angle is formed, setting a direction orthogonal to the X-axis asa Y-axis, setting a region in which the fluorescence luminance detectedby the fluorescence luminance detecting means is equal to or higher thana predetermined value as a non-Facet region, setting a region in whichthe fluorescence luminance is lower than the predetermined value as aFacet region, and setting X-coordinates and Y-coordinates of a boundarybetween the Facet region and the non-Facet region; a laser beamirradiating unit including a condenser that forms a peeling layer inwhich SiC is separated into Si and C and a crack extends along thec-plane, by positioning a focusing point of a laser beam having awavelength transmissible through SiC at a depth corresponding tothickness of a wafer to be produced from the top surface of the SiCingot, and irradiating the SiC ingot with the laser beam; an X-axisfeeding mechanism configured to processing-feed the holding table andthe condenser relative to each other in an X-axis direction; a Y-axisfeeding mechanism configured to indexing-feed the holding table and thecondenser relative to each other in a Y-axis direction; and a controlunit configured to increase energy of the laser beam and raise aposition of the condenser at a time of irradiating the Facet region withthe laser beam with respect to the energy of the laser beam and theposition of the condenser at a time of irradiating the non-Facet regionwith the laser beam on a basis of the X-coordinates and Y-coordinates ofthe boundary between the Facet region and the non-Facet region.